1. Field of the Invention
The present invention relates to prodrugs of therapeutic bisphosphonate compounds and uses thereof to treat or prevent diseases or disorders.
2. Background of the Invention
Clinically used bisphosphonates (BPs) are stable analogs of naturally occurring pyrophosphate (Knight et al., Anticancer Drugs, vol. 16, no. 9, pp. 969-976, 2005; Gnant et al., Curr. Cancer Drug Targets, vol. 9, pp. 824-833, 2009). BPs are known to inhibit cancer cell adhesion and invasion, and inhibit the growth of cancer cells in the bone microenvironment (Boissier et al., Cancer Res., vol. 57, no. 18, pp. 3890-3894, 1997; Boissier et al., Cancer Res. vol. 60, no. 11, pp. 2949-2954, 2000). The two bisphosphonate classes, nitrogen-containing (NBP) and non-nitrogen-containing (NNBP), are distinguished structurally by the substitution pattern at the bridging methylene of the P—C—P linkage. The NBP class incorporates a nitrogen-containing substituent at the bridging methylene (e.g. zoledronate, aledronate, pamidronate) whereas the NNBP class lacks this nitrogen-containing substituent (e.g. clodronate, etidronate) (FIG. 1).
These BP classes are further distinguished by differences in mechanism of action. The NBP class inhibits an essential enzyme in isoprenoid biosynthesis, farnesyl pyrophosphate synthase (FPPS), leading to lower farnesyl pyrophosphate (FPP) levels and subsequent reduction in downstream protein prenylation events in osteoclasts and malignant bone cells. Recent reports suggest that increased levels of the FPPS substrate, IPP, caused by inhibition of FPPS by NBPs, promote formation of AppIPP (triphosphoric acid 1-adenosine-5′yl ester 3-(3-methylbut-3-enyl) ester), which is believed to induce apoptosis (Räikkönen et al., Biochem. Biophys. Res. Commun., Mar. 21, 2011). In contrast, NNBPs undergo conversion to the corresponding non-hydrolyzable ATP analogs. Clodronate is metabolized to the ATP analog AppCCl2p (adenosine 5′-β-γ-dichloromethylene)triphosphosphate), which is believed to be the active metabolite responsible for the apoptotic activity of clodronate in observed osteoclasts and malignant bone cells (Rogers et al., Biochem. J., vol. 303, pp. 303-311, 1994; Frith et al., J. Bone Miner. Res., vol. 12, no. 9, pp. 1358-1367, 1997). Further, AppCCl2p was shown to inhibit mitochondrial metabolism through inhibition of ADP/ATP translocase, and it is conceivable that additional targets are susceptible to inhibition by AppCCl2p (Lehenkari et al., Mol. Pharmacol., vol. 62, pp. 1255-1262, 2002).
Skeletal-related events (SKE) such as fracture, spinal cord compression and hypercalcemia, are common and cause of significant morbidity in cancer patients with bone metastases (Domcheck et al., Cancer, vol. 89, pp. 363-368, 2000). Bisphosphonate therapy has been shown to reduce the rate of SKE in several clinical trials leading to its use as a standard adjunct therapy in patients with bone metastases. The clinical success of NBPs in the prevention and management of bone metastatases has led to the evaluation of BPs as potential therapeutic agents for the treatment of cancer in soft tissues (Morgan et al., Seminars in Oncology, vol. 37, no. 5, pp. S30-S40, 2010). The NBP zoledronate (5, FIG. 1) is a commonly used BP in metastatic bone disease and exhibits varying anti-cancer activities with IC50s ranging from 3 to >100 μM in several cancer cell lines (Knight et al., Anticancer Drugs, vol. 16, no. 9, pp. 969-976, 2005; Morgan et al., Seminars in Oncology, vol. 37, no. 5, pp. S30-S40, 2010; Matsumoto et al., Lung Cancer, vol. 47, no. 1, pp. 31-39, 2005). The cytotoxic effects of zoledronate in cancer cells are believed to be exerted through a variety of mechanisms, including blockage of cell cycle in models of non-small cell lung cancer (Li et al., Lung Cancer, vol. 59, no. 2, pp. 180-191, 2008), inhibition of angiogenesis (Wood et al., J. Pharmacol. Exp. Ther., vol. 302, no. 3, pp. 1055-1061, 2002; Croucher et al., J. Bone Miner. Res., vol. 18, no. 3, pp. 482-492, 2003; Santini et al., Clin. Cancer Res., vol. 13, no. 15, pp. 4482-4486, 2007; Hamma-Kourbali et al., Biochem Biophys Res Commun., vol. 310, no. 3, pp. 816-823, 2003), and induction of apoptosis in small cell lung cancer cell lines (Matsumoto et al., Lung Cancer, vol. 47, no. 1, pp. 31-39, 2005), although the molecular mechanisms beyond inhibition of FPPS are not well-understood.
NNBPs, including clodronate (1, FIG. 1) are significantly less potent anticancer agents (Lipton et al., Cancer Treatment Reviews, vol. 34, pp. 525-530, 2008), exhibiting growth inhibition in the high micromolar or low millimolar range in breast and ovarian cancers (Knight et al., Anticancer Drugs, vol. 16, no. 9, pp. 969-976, 2005), and minimal activity against lung cancer cell lines (Knight et al., Anticancer Drugs, vol. 16, no. 9, pp. 969-976, 2005). The anticancer activity of clodronate is thought to correlate with formation of AppCCl2p in breast, prostate and myeloma cells (Mönkkönen et al., Anticancer Drugs, vol. 19, no. 4, pp. 391-399, 2008); however, the molecular mechanisms underlying the anticancer effects of clodronate are not well-understood.
BPs are polyanionic at physiologic pH, and are consequently concentrated in the mineralized bone matrix (Russell et al., Osteoporosis Int., vol. 19, no. 6, pp. 733-759, 2008). While beneficial for the treatment of bone disorders, this structural characteristic of BPs precludes efficient uptake into extraskeletal tumor cells. The low cellular uptake of BPs presents a critical barrier both for the development of these agents to treat tumors in soft tissues and for studies to elucidate the intracellular mechanisms by which BPs exert anti-tumor effects.
Existing strategies to increase bioavailability of NNBPs such as clodronate have involved masking of the BP scaffold with biodegradable or chemically labile groups designed to release the corresponding BP through non-specific esterase activation or chemical hydrolysis post-intestinal absorption (Ahlmark et al., J. Med. Chem., vol. 42, no. 8, pp. 1473-1476, 1999; Vepsalainen et al., Curr. Med. Chem., vol. 9, pp. 1201-1208, 2002; Zhang et al., J. Med. Chem., vol. 49, no. 9, pp. 5804-5814, 2006). These prodrugs generally undergo rapid extracellular bioactivation in serum, leading to partially unmasked, impermeable intermediates, which are often inefficiently converted to the fully unmasked BP. There are no such prodrug strategies reported for BPs bearing the tertiary hydroxyl group at the bridging methylene position, including the NBP class, owing to the intrinsic instability of these compounds when masked as tetraesters (Neimi et al., Eur. J. Pharm. Sci., vol. 11, no. 2, pp. 173-180, 2000). Other strategies to increase BP cell permeability have focused on introducing modifications at the bridging methylene of the P—C—P linkage to increase hydrophobicity. Such modifications have also been shown to impart changes in target specificity (Szabo et al., J. Med. Chem., vol. 45, no. 11, pp. 2185-2196, 2002; Shull et al., Bioorg. Med. Chem., vol. 14, no. 12, pp. 4130-4136, 2006; Barney et al. Bioorg. Med. Chem. vol. 18, no. 20, pp. 7212-722, 2010). However, increasing hydrophobicity of substituents at the bridging methylene group does not overcome low membrane permeability entirely, as phosphonate masking strategies have been employed in these cases as well (Zhang et al. J. Med. Chem. vol. 49, no. 9, pp. 5804-5814, 2006; Wiemer et al. Bioorg. Med. Chem. vol. 16, no. 7, pp. 3652-3660, 2008).